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Old 08-25-2016, 01:20 PM   #16
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Really interested in the results of this, sub'd!
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Old 08-28-2016, 01:59 AM   #17
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About a week late, but here are the S50B30US crank counter weight radius measurements.
I used the same technique that sert57 used, but with a shitty set of digital calipers.
They are about the same as the 67.75mm quoted by Metric Mechanic.

From front of crank to rear, measured in millimeters.

1 67.75
2 67.93
3 67.93
4 68.02
5 67.85
6 67.93
7 67.85
8 67.91
9 67.88
10 67.92
11 67.88
12 67.85

S50B30US crank weight, no sprocket, spacer or bolt. 22.5Kg

I put the crank in a M20B25 block and the counter weights clear the bottom of the bores and intermediate shaft easily.
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Old 08-28-2016, 08:06 AM   #18
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Quote:
Originally Posted by digger View Post
if you are aworried you can shape the piston to only remove the minimum amount by cutting it into an arc shape rather than full flat bottom skirt
I have done this with no ill effects. Just be sure to weight match the pistons after.

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I put the crank in a M20B25 block and the counter weights clear the bottom of the bores and intermediate shaft easily.
But do the connecting rod big ends/bolts clear as well? They make the swing if the crank a little larger.
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Old 08-28-2016, 02:00 PM   #19
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But do the connecting rod big ends/bolts clear as well? They make the swing if the crank a little larger.
I thought about that as well, but I only have 135mm rods and M20B25 pistons so not much point trying it.
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Old 08-28-2016, 03:04 PM   #20
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its only 0.9 mm extra throw, the rod beam-shoulder transition may touch bottom of bore slighty, nothing a die grinder couldn't fix. ive seen some 89.6 strokers with really wide H-beams that need a huge chamfer on the bottom. Also the rod side /shoulder might kiss the intermediate shaft but probably something a lathe could remedy in 5min.
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Old 09-13-2016, 04:15 PM   #21
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Quick update: I have gotten an S50 crank and gotten it mocked up. As expected, they are snug... ( Once Again, S50b30US crankshaft, Eta 130mm rods, and B25 short skirt pistons)

[IMGhttp://imgur.com/jzmg3K2.jpg[/IMG]



They do clear with no modification! Clearance looks to be about 0.5mm between the counterweight and the skirt. Not exactly what i want by bottom end to look like but that is what lathes are for! Next step will be to get the crank inspected to verify that it is good to use, and take the pistons to a tool shop and hopefully shave off about 5mm off the skirts (until the large boss on the inside). I will keep this thread updated as info rolls in. Fingers Crossed!
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Old 09-13-2016, 06:30 PM   #22
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Take the minimum off you need. 1/8" is plenty clearance, minimum is typically 0.060".

check all to make sure you account for the biggest counterweight
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Last edited by digger; 09-13-2016 at 06:46 PM.
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Old 09-13-2016, 07:07 PM   #23
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Great read. Keep us posted
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Old 09-14-2016, 08:52 AM   #24
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What compression ratios would you be looking at with the stock i pistons, e rods and 885 head?
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Old 09-14-2016, 11:18 AM   #25
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What compression ratios would you be looking at with the stock i pistons, e rods and 885 head?
Its somewhere around 9.5-10:1 Id wager, depending on whether he uses early or late B25 pistons. The B28 stokers can touch 9.5:1 with block machining IIRC...

OP, I'd look into lightening and balancing the full rotating assembly while it's out. I want to do it to my B28 stoker stuff while I can. That way the motor is a little more happy. I'm going with the method I read from Forcedfirebird and basically removing most of the piston skirt anyway. Of course I plan on running a bit more aggressive cam as well, so the theoretical ability to rev the motor a bit higher is appealing to me.
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Old 09-14-2016, 11:24 AM   #26
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Quote:
Originally Posted by Sykohtic View Post
Its somewhere around 9.5-10:1 Id wager, depending on whether he uses early or late B25 pistons. The B28 stokers can touch 9.5:1 with block machining IIRC...

OP, I'd look into lightening and balancing the full rotating assembly while it's out. I want to do it to my B28 stoker stuff while I can. That way the motor is a little more happy. I'm going with the method I read from Forcedfirebird and basically removing most of the piston skirt anyway. Of course I plan on running a bit more aggressive cam as well, so the theoretical ability to rev the motor a bit higher is appealing to me.
Where would the lightening come from? Could taking the counterweights down 1mm then re-balancing the crank get this done? It would reduce the amount that the skirts need to be shaved.
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Old 09-14-2016, 12:06 PM   #27
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Make sure you weight match pistons after.
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Old 09-14-2016, 02:35 PM   #28
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i think one of the main points for the OP was not having to machine the crank counterweights at all. I would massage the pistons for clearance and find a good place and get the assembly balanced and be done with it. if you want it to rev faster then a lighter flywheel is the go
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Old 11-06-2016, 05:32 PM   #29
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Update:
I have finally had a chance to assemble the rotating assembly.
Since I don't have proper machining equipment I went to a very reputable engine machine shop to get the parts machined and balanced ( I will get them to balance the entire assembly when I get a flywheel as well). The block is from a 90 325i automatic with under 100k miles, so it is still in very good shape and I expect it to be within spec. There is no evidence of overheating, cross hatching is still very visible, and there was no ring lap found. I have been assembling it this weekend as much as I can ( the project is on a hold while i wait for new rod and main cap bolts for the final torquing values, as well as piston rings). Anyways, I have been playing with the parts and looking at the clearances via play doh ( and I checked bearings with plastigauge). I am happy to report that it all spins very freely.

Pistons were cut down where needed to clear counterweights. I put some play-doh in the pistons and ran the assembly through a cycle to see what the clearance was. Below is the initial playdoh showing how the pistons were cut down.



Below is the cutaway of the section between the counterweight radius and the piston wrist pin boss. Clearance was measured to be roughly 0.1" which should be enough. Granted, this is not the most precise measurements but i believe they hold some significance.



For the connecting rod to oil pump idler shaft, I performed a similar measurement comparing the b25 rotating assembly to my 2.85L rotating assembly. Below is the result of the 2.85L. Clearance on the B25 was measured to be roughly 0.3", whereas the 2.85L was roughly 0.12"



The final measurement was the location at the bottom of the cylinder bore to the connecting rod shoulder. The B25 results are below with the 2.85L following. The playdoh for the 2.85 actually did get cut at that location, but I believe this was from the removal of the playdoh as opposed to the actual clearance. I roughly measured the B25 clearance to be 0.3" while the 2.85L was roughly 0.1".


B25 -
S50 -

Finally i checked my rod bearings and main bearings with plastigauge. main bearings were all slightly under 0.002", whereas rod bearings were right around 0.0015". This seems to fall right near where Bentley states tolerances are and falls withing the "golden rule" of 0.001" per 1" of journal diameter. I have yet to measure axial play but doubt this will be an issue.
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Old 11-06-2016, 05:34 PM   #30
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I also took a look at the total piston height: 2.85L is below.


I took a shot comparing the 2.85 to the 2.5 but have yet to manipulate it to be meaningful. It looks as though it may be that 0.4mm higher that I expected. I fully expect a custom head gasket to remedy this situation.
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